The mammalian immune system has evolved a variety of mechanisms to protect the host from cancerous cells. An important component of this response is mediated by cells referred to as T cells. Cytotoxic T lymphocytes (CTL) are specialized T cells that primarily function by recognizing and killing cancerous cells or infected cells, but they can also function by secreting soluble molecules referred to as ytokines that can mediate a variety of effects on the immune system. T helper cells primarily function by recognizing antigen on specialized antigen presenting cells, and in turn secreting cytokines that activate B cells, T cells, and macrophages.
A variety of evidence suggests that immunotherapy designed to stimulate a tumor-specific CTL response would be effective in controlling cancer. For example, it has been shown that human CTL recognize sarcomas (Slovin et al., 1986, J Immunol 137, 3042-3048), renal cell carcinomas (Schendel et al., 1993, J Immunol 151, 42094220), colorectal carcinomas (Jacob et al., 1997, Int J Cancer 71, 325-332), ovarian carcinomas (Peoples et al., 1993, Surgery 114, 227-234), pancreatic carcinomas (Peiper et al., 1997, Eur J Immunol 27, 1115-1123), squamous tumors of the head and neck (Yasumura et al., 1993, Cancer Res 53, 1461-1468), and squamous carcinomas of the lung (Slingluff et al., 1994, Cancer Res 54, 2731-2737; Yoshino et al., 1994, Cancer Res 54, 3387-3390). The largest number of reports of human tumor-reactive CTLs, however, has concerned melanomas (Boon et al., 1994, Annu Rev Immunol 12, 337365). The ability of tumor-specific CTL to mediate tumor regression, in both human (Parmiani et al., 2002, J Natl Cancer Inst 94, 805-818; Weber, 2002, Cancer Invest 20, 208-221) and animal models, suggests that methods directed at increasing CTL activity would likely have a beneficial effect with respect to tumor treatment.
Melanoma, or skin cancer, is a disease that is diagnosed in approximately 54,200 persons per year. Conventional therapy for the disease includes surgery, radiation therapy, and chemotherapy. In spite of these approaches to treatment, approximately 7,600 individuals die in the United States every year due to melanoma. Overall, the 5-year survival rate for the disease is 88%. The survival rate drops, however, in more advanced stages of the disease with only about 50% of Stage III patients, and 20-30% of Stage IV patients surviving past five years. In patients where the melanoma has metastasized to distant sites, the 5-year survival dips to only 12%. Clearly, there is a population of melanoma patients that is in need of better treatment options. More recently, in an attempt to decrease the number of deaths attributed to melanoma, immunotherapy has been added to the arsenal of treatments used against the disease.
In order for CTL to kill or secrete cytokines in response to a cancer cell, the CTL must first recognize the cancer cell (Townsend and Bodmer, 1989). This process involves the interaction of the T cell receptor, located on the surface of the CTL, with what is generically referred to as an MHC-peptide complex which is located on the surface of the cancerous cell. MHC (major histocompatibility-complex)-encoded molecules have been subdivided into two types, and are referred to as class I and class II MHC-encoded molecules. In the human immune system, MHC molecules are referred to as human leukocyte antigens (HLA). Within the MHC complex, located on chromosome six, are three different loci that encode for class I MHC molecules. MHC molecules encoded at these loci are referred to as HLA-A, HLA-B, and HLA-C. The genes that can be encoded at each of these loci are extremely polymorphic, and thus, different individuals within the population express different class I MHC molecules on the surface of their cells. HLA-A1, HLA-A2, HLA-A3, HLA-B7, and HLA-B8 are examples of different class I MHC molecules that can be expressed from these loci.
The peptides which associate with the MHC molecules can either be derived from proteins made within the cell, in which case they typically associate with class I MHC molecules (Rock and Goldberg, 1999, Annu Rev Immunol 17, 739-779); or they can be derived from proteins which are acquired from outside of the cell, in which case they typically associate with class II MHC molecules (Watts, 1997, Annu Rev Immunol 15, 821-850). The peptides that evoke a cancer-specific CTL response most typically associate with class I MHC molecules. The peptides themselves are typically nine amino acids in length, but can vary from a minimum length of eight amino acids to a maximum of twelve amino acids in length. Tumor antigens may also bind to class II MHC molecules on antigen presenting cells and provoke a T helper cell response. The peptides that bind to class II MHC molecules are generally twelve to nineteen amino acids in length, but can be as short as ten amino acids and as long as thirty amino acids.
The process by which intact proteins are degraded into peptides is referred to as antigen processing. Two major pathways of antigen processing occur within cells (Rock and Goldberg, 1999, Annu Rev Immunol 17, 739-779). One pathway, which is largely restricted to cells that are antigen presenting cells such as dendritic cells, macrophages, and B cells, degrades proteins that are typically phagocytosed or endocytosed into the cell. Peptides derived in this pathway typically bind to class II MHC molecules. A second pathway of antigen processing is present in essentially all cells of the body. This second pathway primarily degrades proteins that are made within the cells, and the peptides derived from this pathway primarily bind to class I MHC molecules. Antigen processing by this latter pathway involves polypeptide synthesis and proteolysis in the cytoplasm, followed by transport of peptides to the plasma membrane for presentation. These peptides, initially being transported into the endoplasmic reticulum of the cell, become associated with newly synthesized class I MHC molecules and the resulting complexes are then transported to the cell surface. Peptides derived from membrane and secreted proteins have also been identified. In some cases these peptides correspond to the signal sequence of the proteins which is cleaved from the protein by the signal peptidase. In other cases, it is thought that some fraction of the membrane and secreted proteins are transported from the endoplasmic reticulum into the cytoplasm where processing subsequently occurs.
Once bound to the class I MHC molecule, the peptides are recognized by antigen-specific receptors on CTL. Several methods have been developed to identify the peptides recognized by CTL, each method of which relies on the ability of a CTL to recognize and kill only those cells expressing the appropriate class I MHC molecule with the peptide bound to it. Mere expression of the class I MHC molecule is insufficient to trigger the CTL to kill the target cell if the antigenic peptide is not bound to the class I MHC molecule. Such peptides can be derived from a non-self source, such as a pathogen (for example, following the infection of a cell by a bacterium or a virus) or from a self-derived protein within a cell, such as a cancerous cell. The tumor antigens from which the peptides are derived can broadly be categorized as differentiation antigens, cancer/testis antigens, mutated gene products, widely expressed proteins, and viral antigens (Castelli et al., 2000, J Cell Physiol 182, 323-331).
Immunization with melanoma-derived, class I or class II MHC-encoded molecule associated peptides, or with a precursor polypeptide or protein that contains the peptide, or with a gene that encodes a polypeptide or protein containing the peptide, are forms of immunotherapy that can be employed in the treatment of melanoma. This form of immunotherapy requires that immunogens be identified so that they can be formulated into an appropriate vaccine. Although a large number of tumor-associated peptide antigens recognized by tumor reactive CTL have been identified, there are few examples of antigens that are derived from proteins that are selectively expressed on a broad array of tumors, as well as associated with cellular proliferation and/or transformation. Attractive candidates for this type of antigen are peptides derived from proteins that are differentially phosphorylated on serine (Ser), threonine (Thr), and tyrosine (Tyr) (Zarling et al., 2000, J Exp Med 192 1755-1762). Due to the increased phosphorylation of cellular proteins in transformed cells as compared to normal cells, there are likely to be new phosphorylated peptides presented on the cell surface available for recognition by CTL. However, these are not predictable from simple inspection of protein sequences, and the exact phosphorylation sites of many proteins, as well as their phosphorylation state in a tumor cell, remain unknown.
There is a long felt need in the art for methods of identifying tumor antigens, and for methods of treating or preventing cancer based on the use of such tumor antigens. The present invention satisfies these needs.